In microelectronics, bonding generally refers to the interconnection of components by glueing or welding as distinguished from conventional soldering in which the connections are made by means of solder (mostly a tin-lead-alloy). A distinction is made between chip (die) bonding methods for mounting a component on a supporting substrate and wire bonding methods for connecting the component leads to the substrate or for connecting the contact pads of component and substrate by means of fine wires. The present invention deals with the last-mentioned method, in particular with ball bonding, as it is called. The various steps of ball bonding are as follows:
positioning and lowering the bond head or a bond capillary associated therewith and including a flame-scarfed bond ball from an upwardly retracted starting position to a first bonding location; PA0 welding the bond wire ball by supply of heat, defined bonding pressure and/or ultrasonic assistance; PA0 upwardly retracting the bond capillary; PA0 loop formation; PA0 welding the bond wire at a second bonding location by supply of heat, defined bonding pressure and/or ultrasonic assistance; PA0 upwardly retracting the bond capillary to a predetermined tail-length position, breaking the bond wire by subjecting it to a pulling force by means of a clamp engaging the bond wire, and subsequent upward retraction of the bond capillary to the starting position; and PA0 flame-scarfing the free end of the bond wire to form a bond wire ball; PA0 etc. PA0 1. elongation of the often very thin gold wire up to 10%; PA0 2. slippage between the bond wire and the clamp causing breakage thereof; PA0 3. deformation of the bond wire, especially lateral deflection thereof, in the unloaded state after welding to the second bonding pad due to internal stresses.
In view of the progressively increasing miniaturization of components and thus of the bonding or contact pads the size of the ball must be correspondingly reduced so as to prevent short-circuits between adjacent bonding or contact pads. Moreover, there is a demand for the mentioned "mini-balls" to be accurately dimensioned. These two prerequisites cannot readily be satisfied with the described conventional method. This is due to the fact that with conventional ball bonding the following phenomena are not taken into account:
Due to the fact that the specified phenomena are not taken into account, a highly undefined tail length and a correspondingly undefined ball size are obtained with conventional ball bonding.